[Leetcode]543. Diameter of Binary Tree

543. Diameter of Binary Tree

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example:

Given a binary tree 
          1
         / \
        2   3
       / \     
      4   5    
Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.
题意：二叉树的最大直径，
即一个节点的最大直径 = 它左树的高度 + 它右树的高度
思路：
可以将二叉树的直径转换为：二叉树的每个节点的左右子树的高度和的最大值。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
     int res=0;
    int diameterOfBinaryTree(TreeNode* root) {
        dfs(root);
        return res;
    }
    int dfs(TreeNode* root)
    {
        if(root==NULL)
            return 0;
        int left=dfs(root->left);
        int right=dfs(root->right);
        res= max(left+right,res);
        return max(left,right)+1;
    }
};